LONDON: Flights to and from London were severely disrupted on Friday by a technical failure at England's main air traffic control centre that forced authorities to limit access to the country's airspace.

Flights in and out of Heathrow, which calls itself the world's busiest international airport, were halted for at least an hour before it said operations were beginning to resume.

“UK airspace has not been closed, but airspace capacity has been restricted in order to manage the situation,” the National Air Traffic Service (NATS) said on its website.

It later said the system had been restored, but that it would take time for operations to fully return to normal.

Gatwick, another major London airport south of the capital, said flights were departing but with delays. Stansted airport, the city's third busiest, said in a tweet that departing flights were also resuming.

Details on the cause of the problem were not immediately available. A government source, speaking on condition of anonymity, told Reuters it did not appear to be linked to any security threat to the United Kingdom.

As Heathrow is a major hub for international transfers, the disruption was likely to be felt far beyond the UK.

Over 67 million passengers travel through the airport annually on services offered by 90 airlines to over 180 destinations in more than 90 countries, according to its website.

Airports in Birmingham and Manchester, in central and northern England, said they were unaffected by the airspace closure and were ready to accept diverted flights.

British Airways said passengers who did not wish to travel on Friday could seek a full refund or postpone travel.

NATS said it suffered a technical problem at its air traffic control centre in Swanwick, southern England. Swanwick is one of NATS' two main centres. The other is at Prestwick in Scotland.

Mikael Robertsson, co-founder of plane tracking site FlightRadar24, said it was very unusual for hundreds of flights to be diverted at the same time during the Friday afternoon travel peak.

“I can't remember when I saw last something similar. For sure this will affect many thousands of travellers around Europe and the whole world,” he said.

Philippe Guilbert, who was due to fly from Guernsey in the Channel Islands to Gatwick, said the pilot on his plane told passengers he had not heard of similar disruption in Britain in the last 10 years.

Ian Allison, a computing science professor at Robert Gordon University in Aberdeen, said air traffic systems are usually designed to cope with technical failures.

“So it seems to be a major incident that has caused the contingency plans to fail as well as the primary system,” he said.